In recent years, alloy-based negative electrode materials containing elements such as Si (silicon) and Sn (tin) have received attention as negative electrode active materials for nonaqueous electrolyte secondary batteries. Some kinds of metal elements including Si, Sn and the like are capable of absorbing and desorbing Li (lithium) reversibly and electrochemically. Further, as compared with conventional carbon-based materials such as graphite that are generally used as negative electrode active materials, these metal elements have a possibility of increasing a negative electrode capacity. For instance, the theoretical discharging capacity of Si is about 4,199 mAh/g, which is about 11 times the theoretical discharging capacity of graphite.
These alloy-based negative electrode materials, however, tend to expand significantly because the crystal structure of these materials varies because an alloy such as Li—Si or Li—Sn is formed when absorbing Li. In the case where graphite is used as a negative electrode active material, since Li is inserted between layers of graphite (intercalation reaction), the volume expands by about 1.1 times during the Li-absorption. On the other hand, in the case where Si absorbs Li to the maximum extent, the volume is increased by about 4 times theoretically, compared with that prior to the absorption. When a negative electrode active material expands significantly along with the absorption of Li, active material particles may break or an active material layer peels off from a collector, which causes deterioration of the conductivity of the negative electrode. Such a deterioration of the conductivity of the negative electrode leads to deterioration of the battery characteristics such as charging/discharging cycle characteristics.
For instance, as one possible countermeasure for suppressing the peeling-off of an active material layer, the ratio of a binder included in the active material layer may be increased. However, since the binder itself does not contribute to the charging/discharging reaction, the negative electrode capacity thereof may be degraded.
Furthermore, in order to suppress the deterioration of the conductivity of a negative electrode, JP 2002-260637 A for example proposes a negative electrode obtained by sintering in a non-oxidizing atmosphere a mixture of active material particles containing Si and conductive metal powders made of copper or copper alloy on a surface of a collector made of copper foil or copper-alloy foil.
In the negative electrode disclosed by JP 2002-260637 A, however, a Cu—Si compound that does not react with Li electrochemically is produced by the sintering process during the manufacturing, which may degrade the negative electrode capacity. Also, the technology disclosed in the above publication necessarily requires the sintering at a high temperature, where copper (Cu) used as the collector may melt or harden. Such a phenomenon results in the loss of flexibility as the collector, which might make it difficult to configure a battery.